Embodiments of the present invention generally relate to the field of magnetic resonance imaging, and in particular, to a gradient coil for a magnetic resonance imaging apparatus.
The gradient coil of a Magnetic Resonance Imaging (MRI) apparatus provides a fast varying linear field gradient. Designers are constantly seeking ways to improve the efficiency of gradient coils within the constraints of driver compatibility and cost, while trying to maximize bore diameter and openness and minimize length.
The Lorentz force acting on the gradient coils may excite vibration modes and cause significant acoustic noise, which is uncomfortable for the patient and limits the operational performance of the scanner.
According to Faraday's Law, a time-varying magnetic field will induce an electric field E, which will, in turn, induce electric current in conducting structures. This may cause Peripheral Nerve Stimulation (PNS) or painful atrial fibrillation (which may be life-threatening)
In prior art, there are some gradient coil geometries that may be used to improve some aspects of performance or reduce acoustic noise; however, these geometries can be more difficult to manufacture or may compromise performance in other aspects.
U.S. Pat. No. 5,561,371 describes a folded gradient coil that can reduce the length of the coil and improve the performance of a transverse gradient coil by folding the return arc wires upon the forward arc wires. However, such coils are difficult to manufacture and there is a need to have multiple connections between the primary turns and the shield turns might cause reliability concerns. Furthermore, it would be extremely difficult to build this type of coil with hollow conductor.
U.S. Pat. No. 5,554,929 describes a crescent-shaped gradient coil with compact size and low acoustic noise. However, these coils are constrained to have the same number of the turns in the primary and shield making them inherently over-shielded, thus causing unwanted interactions with the magnet.
In Blaine A. Chronik, Andrew Alejski and Brian K. Rutt's “Design and Fabrication of a Three-Axis Edge ROU Head and Neck Gradient Coil”, MRM, 44:955-963 (2000), an asymmetric gradient coil is described. While an asymmetric geometry may bring the Field of View (FOV) much closer to one end of the gradient coil, the lack of symmetry would pose many challenges including unwanted electromagnetic coupling and torque acting on the coil.
U.S. Pat. No. 6,921,042B1 describes a double helix magnet and possibility to create multi-polar fields. Concentric tilted double-helix magnets are obtained by winding complete tilted conductors on a core. U.S. Pat. No. 7,889,042B2 describes a particular implementation of the above with varying conductor width. While it is possible to create a quadrupole field with this configuration, such complete windings on a single core result in geometries with a long length and high impedance.
Therefore, there is a need for a novel geometry of a gradient coil that may improve the performance of the gradient coil, allow increased diameter and or openness of the bore, reduce the acoustic noise and reduce the tendency to cause PNS, without increasing the manufacturing costs, prohibiting the use of hollow conductor or compromising the performance in other aspects.